U.S. Pat. No. 5,386,253 to Fielding, incorporated herein in its entirety by this reference, discusses exemplary projection systems utilizing one or more spatial light modulators (SLMs). As noted in the Fielding patent:                Spatial light modulator devices include so-called “active matrix” devices, comprising an array of light modulating elements, or “light valves,” each of which is controllable by a control signal (usually an electrical signal) to controllably reflect or transmit light in accordance with the control signal. A liquid crystal array is one example of an active matrix device; another example is the deformable mirror device (DMD) developed by Texas Instruments . . .See Fielding, col. I, II. 13–21. Of course, yet other types of light “engines,” or sources, exist, and various of them may be used in connection with the inventions described herein.        
Regardless of the type of light sources and modulators used, audiences frequently desire to see images high in detail and richness and low in objectionable artifacts. High resolution and image quality in particular facilitates suspension of disbelief of an audience as to the reality of the projected images. Such quality indeed often is an important factor in the overall success of the motion picture viewing experience among today's audiences.
Producing these high-resolution images is not without added cost, however. Imax Corporation, for example, the intended assignee of this application, utilizes not only specialized cameras and projectors, but also seventy millimeter, fifteen perforation film to increase the resolution and quality of projected images. Conventional electronic projectors (and especially those utilizing SLMs), by contrast, generally cannot supply equivalent resolution in projected images. As well, such electronic projectors frequently fail to furnish the dynamic range and overall brightness of images provided by large-format films. They nonetheless may desirably (or necessarily) be employed to display non-film-based images such as (but not limited to) computer-generated graphics or material captured with electronic cameras.
A DMD is a type of SLM that consists of a two dimensional array of mirrors. The mirror array is imaged through a projection lens onto a screen so that each mirror functions as an image pixel. Each mirror can be electronically controlled to assume two positions, one that reflects incident light towards the projection lens, this is the “on” state, and another position that does not reflect incident light towards the projection lens but directs it instead to for example a beam dump, this is the “off” state.
The DMD is therefore a binary light modulator. Variable intensity may be produced by controlling the time that a mirror spends in each state, on or off, and repeatedly cycling each mirror between the on and off states in a regular pattern according to a series of image frames as is conventional in the display of moving images. By varying the amount of time each mirror spends in the on state during each frame time the brightness of each pixel can be controlled. This technique is called pulse width modulation or PWM.
Using PWM a grayscale can be created with a DMD device. This grayscale can be controlled by input digital data in the form of a binary code. For example, dividing each frame time into ten time periods of different lengths can create a 10 bit gray scale. The length of the time period corresponding to the least significant bit (LSB) in the address signal for any particular frame is set at a predetermined value, the duration of the time period corresponding to the next significant bit (LSB+1) being twice as long as that corresponding to the LSB and so on. Thus, the length of the time period corresponding to the most significant bit (MSB) for a 10 bit input signal is 512 times that corresponding to the LSB. This gives a total of 1024 possible gray scale values between full black (the DMD mirror remains in the off state for the fill frame time) to full white (the DMD mirror remains in the on state for the full frame time). Provided that the lowest PWM frequency for the MSB is above the “fusion frequency” for the human visual system, each of the PWM cycles will be integrated and provide the sensation of a continuously variable grayscale corresponding to the binary value of the input signal. This technique is called binary PWM.
Using binary PWM the output brightness level from each mirror is in proportion to the fraction of time that the mirror is “on” within a frame interval. As a result, the output brightness level B from a single DMD pixel can be modeled by the following equation:B=(αy+δ)TL=αyTL+δTL.  (1)
In equation (I), L is the incident light intensity from a light source, y is the digital signal with normalized values ranging from 0 to 1 and T is the time duration of each display frame. The factor α<1 represents the optical efficiency of a DMD pixel. The maximum output or “white level” from a DMD device is obtained when signal value reaches its maximum or y=1, i.e.:Bw=By=1=(α+δ)TL.  (2)
Similarly, the minimum output or “dark level” of the DMD device is reached when y=0, i.e.:Bb=By=0=δTL.  (3)
The ratio of maximum to minimum level determines the contrast ratio of a DMD-based projector. The minimum level is the result of unwanted light being reflected into the projection lens pupil when the mirror is in the off state. This is caused by several factors including scattering from the mirror edges and the structure beneath the mirrors. The sources of unwanted off state light are combined into the term δ in Equation (1). For a DMD device that truly supports an n-bit dynamic range, its dark level must be less than the brightness level represented by the least significant bit (LSB) of the digital input signal. In other words, the following relationship must be maintained:
                    δ        <                              1                                          2                n                            -              1                                .                                    (        4        )            
The dark level or the lowest light level that can be displayed sets a limit to the amount of detail that can be generated in dark scenes. In the case of a DMD projector system, the switching speed of the DMD device determines the minimum bit time (LSB). In addition, the dark level represents the minimum displayable level when the DMD is in the fully off state. Reducing the LSB display brightness below some critical value produces little gain in apparent gray scale bit depth since the increase in grayscale resolution is masked by the DMD dark level. The dark level also limits the contrast ratio that the system can display. For typical SLM based projection systems this is between 200:1 and 500:1 depending on the optical design.
In order for a viewer to perceive images that have a full range of tones, allowing the richest imagery that is as close to reality as possible it is necessary to provide varying levels of projection system contrast depending on the light levels that are provided by the system and also depending on the ambient light levels of the surrounding viewing environment. The human visual system has a “simultaneous contrast range” which refers to the contrast range that a typical human observer can see at one time in a given state of adaptation to overall scene brightness. This is normally accepted to be in the range of 200:1. However, the human visual system adapts its simultaneous contrast range to a much wider range of total scene brightnesses, amounting to about seven decades from the darkest part of the scene to the brightest. It is common for an observer to change adaptation over a significant portion of this range as the observer's point of regard in the scene changes. This is exemplified by looking at the exterior of a building in bright sunlight, and then looking into the underground parking lot. A typical viewer can see both cars in the parking lot and features on the bright building even though the total contrast range in this scene exceeds the simultaneous contrast range that the viewer can perceive.
The projection system contrast that is required to produce a sensation equivalent to a full range of tones increases as the projection light levels decrease, and also increases as the surround becomes darker. In a typical motion picture theatre viewing environment a projection contrast of 1000:1 or higher is needed in order for the viewer to perceive a full range of tones equivalent to the viewer's simultaneous contrast range of 200:1. In addition, the size of the steps in grayscale that are required for a difference to be perceived varies with brightness. An observer can discriminate between much smaller steps in grayscale at lower levels of brightness than he/she can at a higher brightness level.
When representing grayscales using binary data it is common to refer to the number of bits in the binary numbers used as the “bit depth.” A greater number of bits obviously produce finer steps in the gray scale, and up b some threshold, determined in part by the viewing conditions, a larger number of gray scale steps, and therefore a larger number of bits are desirable. However, as discussed above, there is no value in subdividing the grayscale steps below the smallest step that is just perceivable above the dark level of the projection system.
In International Patent Application WO 94/10675 (incorporated herein in its entirety by reference), there is described a method of increasing the bit depth of a DMD based display system in which the intensity of the light source used to illuminate the DMD is modulated on a binary basis. However, while extending the normal gray scale bit depth (since this binary modulation takes place within a single video frame) it has no effect on the DMD black level. Also with such lamp modulation, the power supply has to change output very rapidly and thus imposes demanding design requirements on the lamp power supply and may also generate a significant amount of electromagnetic interference.
The dark level can be reduced as improvements are made to the architectural design of DMDs and other light modulating devices, but it may not be completely eliminated. Therefore, equipment and techniques for decreasing the dark level and thus, increasing the dynamic range of a SLM projector are desirable.